Recently, as environmental concerns have moved to the forefront, there has been a push to provide a more efficient and cleaner form of energy. One proposed solution has been the fuel cell. The fuel cell is an electrochemical energy conversion device that converts fuel and oxygen into electricity, heat, and innocuous by-products such as water vapor. Emissions from the fuel cell system typically are significantly smaller than emissions from the cleanest fuel combustion processes.
A fuel cell system is made up of a number of individual fuel cells that form a fuel cell “stack.” Fuel can be supplied to the fuel cell stack in a number of ways. For example, a proton exchange membrane (PEM) fuel cell can be fed directly from a source of hydrogen, or it can operate from hydrogen that is being supplied from a fuel reformer. Typically, the air required by most fuel cells is pumped to the fuel cell stack at a rate that varies with the load and/or various operating conditions. These other devices necessary to operate the fuel cell system (e.g., pumps and fans) are called the system's “balance-of-plant.” The balance-of-plant components and the reformer typically cause a fuel cell stack to respond to the inevitable load changes much slower than batteries (i.e.,—the balance-of-plant and reformer are unable to keep up with instantaneous changes in the load).
Each individual cell in a fuel cell stack produces direct-current (DC) energy, typically with a high current and a low voltage (e.g., 0.7 volts). The low DC voltage produced by the fuel cell varies with the operating conditions such that the voltage is highest at no load and lowest at full load. A typical ratio between full-load and no-load voltage may be 2 or more. The DC energy produced by the fuel cell may be used both in stationary and mobile applications. Certain applications, for example residential and commercial loads, require an alternating current (AC) output. As a result, fuel cells, like many other alternative DC energy sources (e.g., solar energy) require an inverter to convert the DC voltage into AC voltage. Once converted, this AC source may be used as a stand-alone source and/or in parallel with the electrical power transmission grid that currently provides power to residential and commercial loads.
Fuel cells place many unusual constraints on the inverter device that is responsible for converting the fuel cell system's output to a regulated AC voltage. For example, the inverter must be able to adapt to the varying output voltage of the fuel cell. Also, the inverter must protect the fuel cell from a reverse current or an unstable input current, both of which could destroy the fuel cell. In addition, because the fuel source typically is incapable of instantaneously responding to the varying demands of the load, the inverter must be able to cooperate with a DC storage source (e.g., a battery) as well as the fuel cell. As a result of these differences, traditional power inverters can not satisfy the requirements of the fuel cell system, particularly in stand-alone applications (i.e., where the fuel cell inverter directly powers the load).
Therefore, it would be advantageous to provide a high efficiency DC-to-AC inverter suited to accommodate the unique operation of the fuel cell powered system.